We have molecularly cloned M432 retroviral and intracisternal A particle (IAP) genes from M. cervicolor cellular DNA. The 4 kbp major region of homology between the M. musculus and M. cervicolor IAP genes does not include the long terminal repeat regions of the genomes. The molecularly cloned M. cookii type C-I proviral genome was found to share significant sequence homology to the gag-pol region of the woolly monkey infectious primate type C viral genome. This is consistent with previous immunological results, suggesting that the origin of the woolly monkey-gibbon ape group of infectious primate type C viruses is from this class of endogenous murine type C viruses. Sequences related to the type C-I class of endogenous retroviral genome are highly conserved in all species of Mus. We have found that in M. musculus, but not in other murine species, at least 100 copies of type C-I-related sequences are located on the Y chromosome.